drm/i915/crt: Ensure the HW is powered during HW state readout

drm/i915/crt: Ensure the HW is powered during HW state readout

The assumption when adding the intel_display_power_is_enabled() checks
was that if it returns success the power can't be turned off afterwards
during the HW access, which is guaranteed by modeset locks. This isn't
always true, so make sure we hold a dedicated reference for the time of
the access.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93439
CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455296121-4742-7-git-send-email-imre.deak@intel.com
(cherry picked from commit 1c8fdda1ea947ae8cf994969a1c285acc7089cb9)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>